The supply of counterfeit goods in a particular market causes a loss of revenue to manufacturers of the corresponding genuine goods, as well as to governments when those goods are subject to taxation. End users are adversely affected by counterfeit goods because they are supplied with products of inferior quality, which may even be dangerous to the health of the end user for certain products, such as when medicines are the subject of counterfeiting. The manufacturer of high-quality genuine products will consequently suffer a loss to its goodwill.
A number of anti-counterfeiting measures have been proposed in the prior art with respect, for example, to alcoholic and non-alcoholic drinks (beer, wine, liquor, soft-drinks, etc.), tobacco products (cigarettes, cigars, loose tobacco, etc.), medicinal products, perfumes and excisable products generally. It is known to make use of sophisticated printing techniques to make the design on the package as hard to replicate as possible.
It is also known to make use of fluorescing items that look one way under ambient light and look a different way under ultraviolet (UV) radiation. Also used are holographic images of varying degrees of complexity. Other known techniques include watermark technology, engraved gravure lines and marks that change colour depending on heat applied to the mark.
CN 202533362 U relates to a printed matter authenticity identification device based on a multispectral imaging technology. The device comprises a multispectral imager for carrying out multispectral scanning on a test sample (the multispectral imager comprising a light source, a grating, and an imaging detector), a spectral data processor for comparing spectral data obtained from scanning with spectral data of a standard sample, and a data server used for storing the spectral data of the standard sample. If the difference between the spectral data obtained from scanning and the spectral data of a standard sample exceeds a set threshold value, the test sample is judged as fake. Otherwise, it is judged as authentic.
The prior art also includes various imaging spectrometers used for scientific observations. These systems typically aim at obtaining high-resolution spatial and spectral information about all regions of a scene or object. In particular, imaging spectrometers are imagers that allow extraction of three-dimensional spectral irradiance map of a planar object (spatial-spectral data cube) I(x, y, A) by use of two-dimensional array detectors such as CCD (i.e., charge-coupled device) or CMOS (i.e., complementary metal-oxide-semiconductor) sensors. One dimension is the wavelength and the other two comprise the spatial information.
Two major categories of spectral imagers exist: the spectral scanning imagers and the snapshot spectral imagers. A review of multi- and hyperspectral imager can be found for example in Hagen et al, “Snapshot advantage: a review of the light collection improvement for parallel high-dimensional measurement systems”, Optical Engineering 51(11), 111702 (2012), and Hagen et al, “Review of snapshot spectral imaging technologies”, Optical Engineering 52(9), 090901 (September 2013).
One way to acquire three-dimensional information by a two-dimensional sensor is to acquire sequentially images through mechanically scanned wheel or array of optical filters installed in front of an imager. Another possibility is to tune the central transmission band of a filter such as a multi-stage liquid crystal filter, an acousto-optic filter, or a Fabry-Perot interferometer. These two examples fall into the category of spectral scanning imagers.
Snapshot spectral imagers capable of simultaneous acquisition of images in different spectral bands through an array of filters exist and an example is the multi-aperture filtered camera (MAFC), using lenslet arrays with focal plane detector.
Transmission diffraction gratings based snapshot spectral imaging systems also exist. An example is the computed tomography imaging spectrometer (CTIS) which either uses several crossed transmission gratings or specifically designed Kinoform grating able to disperse several spectral orders around a zero order. Computed tomography algorithms have to be used to reconstruct the spectral radiance of the object.
Another example with transmission diffraction grating is the coded aperture snapshot spectral imager (CASSI) which uses complex masks to shadow some parts of the image of the object in order to facilitate the spectra extraction.
Integral field imaging spectrometers rely also on diffraction gratings to disperse the light. In these setups, the image is sliced by different methods to fit onto an input slit of a conventional spectrometer to extract spectra. Image slicing can be obtained either by use of fiber bundle and distributing individual fibers into an entrance slit, or by aperture division using lenslet array.
Fourier transform imaging spectrometers also exist in a separate category. An interferometer is scanned to obtain images at different optical path differences and spectra are reconstructed by Fourier transform. Some setups rely on lenslet array to do aperture division and analyse the average spectra at different parts of the image/object. An example is the multiple-image Fourier transform spectrometer (MIFTS) based on a Michelson interferometer. Another distinct example is the snapshot hyperspectral imaging Fourier transform spectrometer (SHIFT) which uses pair of birefringent prisms to obtain different optical path lengths.
In view of the above, there is a need for providing fast, simple, inexpensive, compact, and robust equipment for authentication purposes, in particular, but not only, for incorporation into hand-held audit devices.